A search for t[bar over t] resonances in lepton+jets events with highly boosted top quarks collected in pp collisions at √s = 7 TeV with the ATLAS detector The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Aad, G., T. Abajyan, B. Abbott, J. Abdallah, S. Abdel Khalek, A. A. Abdelalim, O. Abdinov, et al. “A search for t[bar over t] resonances in lepton+jets events with highly boosted top quarks collected in pp collisions at √s = 7 TeV with the ATLAS detector.” J. High Energ. Phys. 2012, no. 9 (September 2012). © CERN, for the benefit of the ATLAS collaboration As Published http://dx.doi.org/10.1007/jhep09(2012)041 Publisher Springer-Verlag Version Final published version Citable link http://hdl.handle.net/1721.1/86206 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/3.0/ Published for SISSA by Springer Received: July 10, 2012 Accepted: August 16, 2012 Published: September 13, 2012 JHEP09(2012)041 A search for tt¯ resonances in lepton+jets events with highly boosted top quarks collected in pp collisions at p s = 7 TeV with the ATLAS detector The ATLAS collaboration E-mail: [email protected] Abstract: A search for resonant production of high-mass top-quark pairs is performed p on 2.05 fb−1 of proton-proton collisions at s = 7 TeV collected in 2011 with the ATLAS experiment at the Large Hadron Collider. This analysis of the lepton+jets final state is specifically designed for the particular topology that arises from the decay of highly boosted top quarks. The observed tt¯ invariant mass spectrum is found to be compatible with the Standard Model prediction and 95% credibility level upper limits are derived on the tt¯ production rate through new massive states. An upper limit of 0.7 pb is set on the production cross section times branching fraction of a narrow 1 TeV resonance. A Kaluza-Klein gluon with a mass smaller than 1:5 TeV is excluded. Keywords: Hadron-Hadron Scattering Open Access, Copyright CERN, doi:10.1007/JHEP09(2012)041 for the benefit of the ATLAS collaboration Contents 1 Introduction1 2 The ATLAS detector2 3 Data and Monte Carlo samples3 4 Physics object reconstruction and selection5 JHEP09(2012)041 5 Event selection and reconstruction of the tt¯ system6 6 Estimate of the multijet background from data9 7 W +jets production normalization and control regions 11 8 Systematic uncertainties 11 9 Comparison of data to the Standard Model prediction 14 10 Interpretation 14 11 Summary 19 The ATLAS collaboration 24 1 Introduction The Large Hadron Collider (LHC) opens up a new kinematic regime, where pairs of Stan- dard Model (SM) particles can be produced with an invariant mass of several TeV. Such high-mass final states are of particular interest for searches for massive states predicted by a number of extensions of the Standard Model. High-mass pairs of top quarks are among the most interesting of the final states explored by the ATLAS [1] experiment, but also represent a considerable experimental challenge. The topology that forms when these Lorentz-boosted top quarks decay differs in impor- tant respects from that encountered when top quarks are produced approximately at rest. New tools have been developed to fully exploit the potential of these states. We adopt a so- lution proposed by Seymour [2] for the reconstruction and identification of highly boosted, hadronically decaying, massive particles, where these boosted objects are reconstructed as a single fat jet. An overview of the tools developed for the reconstruction of boosted objects is found in refs. [3,4 ]. { 1 { In this paper results are presented of a resonance search in the lepton+jets final state ¯ 0 that arises in the reaction pp ! tt¯ ! bbqq¯ `ν`, where one of the W bosons from the top quarks decays to a charged lepton and a neutrino, and the other to jets of hadrons. Events are classified as belonging to the \e+jets" or \µ+jets" channel, depending on whether the charged lepton is an electron or a muon. We search for the distinct shape of a resonant signal in the reconstructed tt¯ invariant mass distribution. Compared to searches for tt¯ resonances carried out by the CDF [5{9] and D0 [10, 11] collaborations at Run II of the Fermilab Tevatron Collider and a previous search by AT- LAS [12] using the present data set, this analysis is specifically designed for top-quark JHEP09(2012)041 pairs with an invariant mass beyond 1 TeV [13]. Jets are reconstructed with the anti-kt algorithm [14] with a larger radius parameter (R = 1:0) than is usually employed in AT- LAS. The highly energetic top quark decaying to three jets of hadrons (t ! W b ! bqq¯0) is reconstructed as a single fat jet. The selection relies strongly on an analysis of the jet substructure. Also the reconstruction of the second top quark candidate (with the decay t ! W b ! b`ν`) relies on the large boost of the top quarks; the jet assignment is based on the vicinity to the charged lepton originating from the top quark decay. While tt¯ resonance searches are relevant for any extension of the Standard Model that leads to an enhanced top quark pair production rate at large tt¯ invariant mass, we interpret the result within two specific benchmark models. The leptophobic topcolor Z0 boson1 [15] represents an example of a narrow resonance, where the experimental resolution dominates the width of the reconstructed mass peak. The Tevatron searches have set a 95% credibility 0 level (CL) limit on the mass of the leptophobic topcolor Z boson [16] at mZ0 > 900 GeV [8]. The second benchmark model envisages a Kaluza-Klein (KK) excitation of the gluon gKK , as predicted in models with warped extra dimensions [17, 18]. For the choice of parameters of Lillie et al. [19] used here, the KK gluon manifests itself as a relatively broad resonance (Γ=m = 15:3%) with a branching fraction BR(gKK ! tt¯) = 92:5%. The first tt¯ resonance searches on LHC data [12, 20] exclude Kaluza-Klein gluons [17, 18] with a mass smaller than 1.13 TeV [12] at 95% CL. 2 The ATLAS detector The ATLAS detector [1] is a multi-purpose particle detector with a forward-backward symmetric cylindrical geometry and almost 4π coverage in solid angle. The inner detector (ID), composed of a silicon pixel detector, a silicon microstrip de- tector and a transition radiation tracker, provides efficient reconstruction of the trajectories of charged particles in the pseudorapidity2 range up to jηj = 2:5. 1 The specific case considered here corresponds to model IV in ref. [15] with f1 = 1 and f2 = 0 and a width of 1.2% of the Z0 boson mass. 2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis along the beampipe. The azimuthal angle φ is measured with respect to the x-axis, which points towards the centre of the LHC ring. The y-axis points up. The pseudorapidity η is defined in terms of the polar angle θ as η = − ln tan θ=2. The transverse momentum pT is defined as pT = p sin θ. { 2 { The ID is surrounded by a thin superconducting solenoid producing a 2 T magnetic field and by a hermetic calorimeter system, which provides three-dimensional reconstruction of particle showers up to jηj = 4:9. A highly granular lead and liquid-argon (LAr) sampling calorimeter provides a precise measurement of the energy of electrons and photons. The hadronic calorimeter uses steel and scintillating tiles in the barrel region (jηj < 1:7), while the endcaps use LAr as the active material and copper as absorber. The forward calorimeter (jηj > 3:1) also uses LAr as the active medium, with copper and tungsten as absorber. The muon spectrometer consists of one barrel and two endcap air-core toroidal mag- nets, each consisting of eight superconducting coils arranged symmetrically in azimuth around the calorimeter. Three layers of precision tracking chambers, consisting of drift JHEP09(2012)041 tubes and cathode strip chambers, allow precise muon momentum measurement up to jηj = 2.7. Resistive plate and thin-gap chambers provide muon triggering capability up to jηj = 2.4. The trigger system is composed of three consecutive levels. The level-1 trigger is based on custom-built hardware that processes coarse detector information to reduce the event rate to a design value of at most 75 kHz. This is followed by two software-based trigger levels, level-2 and the event filter, which together reduce the event rate to a few hundred Hz which is recorded for analysis. 3 Data and Monte Carlo samples The data used in this search were collected by the ATLAS detector at the LHC in 2011 using a single-muon or single-electron trigger with transverse momentum (pT) thresholds set at 18 GeV for muons and 20 GeV or 22 GeV for electrons. The object requirements used in the offline selection are more stringent than those used in the trigger, and the of- fline pT thresholds are chosen on the efficiency plateau for the trigger. Only data recorded under stable beam conditions between March and August 2011 are used. Moreover, all subdetectors are required to be operational. The resulting data sample corresponds to an integrated luminosity of 2:05 ± 0:08 fb−1 [21, 22]. Simulated samples are used to predict the contribution of the Standard Model back- grounds, the most important of which are tt¯ production, vector boson production in asso- ciation with jets and multijet production.